An automotive automatic transmission includes a multiple-disc clutch, provided with alternately arranged friction discs and separator plates, and immersed in an automatic transmission fluid (ATF) serving as a lubricating oil. The friction plates and the separator plates are frictionally engaged by being pressed against one another to transmit power, or disengaged, by removing engagement pressure, to intercept power transmission.
Although some conventional brake drums, are formed of gray cast iron, generally, brake drums are formed of steels, i.e., carbon steels, superior in strength and toughness to gray cast iron. A separator plate of a wet type multiple-disc clutch or a multiple-disc brake is formed by punching a steel sheet, such as a cold-rolled steel sheet, to produce an annular plate, deburring and grinding the annular plate, and finishing the deburred annular plate by a polishing process, such as barrel finishing.
In the initial stage of use, before the friction members and the friction mating members have adapted to each other, the friction coefficient between the friction members and the friction mating members is unstable, and the friction device exhibits a phenomenon known as “shudder,” sometimes referred to as “judder,” characterized by the generation of noise and vibration as the friction members and the friction mating members become engaged with one another.
Frictional vibration is the principal cause of shudder, and the dependence of the friction coefficient on speed (the so-called “μ-V characteristic”) is considered to have a strong influence on frictional vibration. Generally, the occurrence of shudder can be almost entirely avoided when the μ-V characteristic curve, indicating the variation of the friction coefficient with speed, has a positive gradient, that is, increased friction in the high-speed range and decreased friction in the low-speed range.
Various ways have been proposed to provide a friction device with an μ-V characteristic having a positive gradient. One proposal for improving the μ-V characteristic was to increase the roughness of the surface of a friction mating member by barrel finishing to increase the friction coefficient in the high speed range. However, the friction coefficient in the low-speed range is highly dependent on the effect of the friction modifier (FM) contained in the lubricating oil, and the friction coefficient changes when the lubricating oil deteriorates or when the lubricating oil is changed. Thus, when the μ-V characteristic is affected adversely by the lubricating oil, a positive gradient in the μ-V characteristic of a friction device cannot necessarily be achieved solely by increasing the friction coefficient in the high-speed range.
Another known contact part has a hard surface coating having a smooth surface with minute roughness. The surface of a friction member used in combination with this known contact part is finely polished, and the smoothness of the surface of the friction member is improved as the contact part and the friction member slide relative to each other. The apparent contact area of the friction member is increased, and the surface of the friction member is coated with a uniform, thin, oil coating. Consequently, the friction coefficient between the friction mating member and the friction member in a high-speed range is increased.
A friction mating member having such a hard coating is formed by applying, to a base member, a coating of amorphous carbon or a ceramic material, such as TiN, having an irregular thickness and a distributed particle size. Such a friction mating member is disclosed in Unexamined Japanese Patent Publication No. 366029/1992. Another friction mating member, disclosed in Unexamined Japanese Patent Publication No. 272517/1992, is formed by coating a base member, having a surface finished by lapping, with a TiN coating.
As is generally known, a carbonaceous coating material, such as amorphous carbon, is chemically stable. Hence, it is difficult for an additive contained in a lubricating oil to form a boundary coating on the surface of the carbonaceous coating. It is known that a carbonaceous coating is satisfactory in abrasion resistance, and exhibits a low friction coefficient, in the range of 0.1 to 0.2, under unlubricated conditions in a low-speed range, owing to its own self-lubricating characteristic and physical properties.
The μ-V characteristics of the friction mating members mentioned in the above-identified patent documents are improved to some extent, but are still not entirely satisfactory. This invention addresses the foregoing problems. It is an object of the invention to provide a friction mating member having an improved μ-V characteristic, in which the friction coefficient in the low-speed range is reduced without being affected by the type of lubricating oil or by deterioration of components of the lubricating oil, and in which the friction coefficient is increased in the high-speed range.